public class ArcGISPortalManager 

// A portal project path looks like this:
//@"https://<ServerName>.<Domain>.com/portal/sharing/rest/content/items/1a434faebbe7424d9982f57d00223baa";
//A local project path looks like this:
//@"C:\Users\<UserName>\Documents\ArcGIS\Projects\MyProject\MyProject.aprx";

//Check if the project can be opened
if (Project.CanOpen(projectPath, out docVer))
{
  //Open the project
  await Project.OpenAsync(projectPath);
}
else //The project cannot be opened
{
  //One possible reason: If the project is a portal project, the active portal must match the portal of the project
  //Check if this is a portal project
  bool isPortalProject = Uri.TryCreate(projectPath, UriKind.Absolute, out Uri uriResult)
       && (uriResult.Scheme == Uri.UriSchemeHttp || uriResult.Scheme == Uri.UriSchemeHttps);

  if (isPortalProject)
  {
    //Parse the project path to get the portal
    var uri = new Uri(projectPath);
    var portalUrlOfProjectToOpen = $"{uri.Scheme}://{uri.Host}/portal/";

    //Get the current active portal
    var activePortal = ArcGIS.Desktop.Core.ArcGISPortalManager.Current.GetActivePortal();
    //Compare to see if the active Portal is the same as the portal of the project
    bool isSamePortal = (activePortal != null && activePortal.PortalUri.ToString() == portalUrlOfProjectToOpen);
    if (!isSamePortal) //not the same. 
    {
      //Set new active portal to be the portal of the project
      //Find the portal to sign in with using its Uri...
      var projectPortal = ArcGISPortalManager.Current.GetPortal(new Uri(portalUrlOfProjectToOpen, UriKind.Absolute));
      await QueuedTask.Run(() => {
        if (!projectPortal.IsSignedOn())
        {
          //Calling "SignIn" will trigger the OAuth popup if your credentials are
          //not cached (eg from a previous sign in in the session)
          if (projectPortal.SignIn().success)
          {
            //Set this portal as my active portal
            ArcGISPortalManager.Current.SetActivePortal(projectPortal);
            return;
          }
        }
        //Set this portal as my active portal
        ArcGISPortalManager.Current.SetActivePortal(projectPortal);
      });
      //Now try opening the project again
      if (Project.CanOpen(projectPath, out docVer))
      {
        await Project.OpenAsync(projectPath);
      }
      else
      {
        System.Diagnostics.Debug.WriteLine("The project cannot be opened.");
      }
    }
    else //The portals are the same. So the problem could be something else - permissions, portal is down?
    {
      System.Diagnostics.Debug.WriteLine("The project cannot be opened.");
    }
  }
  else //Project is on disk and cannot be opened. 
  {
    System.Diagnostics.Debug.WriteLine("The project cannot be opened.");
  }
}

var projectPortal = ArcGISPortalManager.Current.GetPortal(new Uri(@"https://<serverName>.<domain>.com/portal/", UriKind.Absolute));
string owner = string.Empty;
await QueuedTask.Run(() => {
  //Get the signed on user name
  owner = projectPortal.GetSignOnUsername();
});
//Get the user content from the portal
var userContent = await projectPortal.GetUserContentAsync(owner);
//Get the first portal project item
var firstPortalProject = userContent.PortalItems.FirstOrDefault(pi => pi.PortalItemType == PortalItemType.ProProject);
var portalProjectUri = firstPortalProject.ItemUri.ToString();
//Check if project can be opened
string docVer = string.Empty;
if (Project.CanOpen(portalProjectUri, out docVer))
{
  await Project.OpenAsync(portalProjectUri);
}
//Note: If Project.CanOpen returns false, the project cannot be opened. One reason could be 
// the active portal is not the same as the portal of the project. Refer to the snippet: [Workflow to open an ArcGIS Pro project](ProSnippets-sharing#workflow-to-open-an-arcgis-pro-project)

var active_portal = ArcGISPortalManager.Current.GetActivePortal();
string uri = active_portal.PortalUri.ToString();

var portals = ArcGISPortalManager.Current.GetPortals();
//Make a list of all the Uris
var portalUris = portals.Select(p => p.PortalUri.ToString()).ToList();

var portalUri = new Uri("http://myportal.esri.com/portal/", UriKind.Absolute);
ArcGISPortalManager.Current.AddPortal(portalUri);

//Find the portal to sign in with using its Uri...
var portal = ArcGISPortalManager.Current.GetPortal(new Uri(uri, UriKind.Absolute));
if (!portal.IsSignedOn())
{
  //Calling "SignIn" will trigger the OAuth popup if your credentials are
  //not cached (eg from a previous sign in in the session)
  if (portal.SignIn().success)
  {
    //Set this portal as my active portal
    ArcGISPortalManager.Current.SetActivePortal(portal);
  }
}